Straub, a young scientist in
Szent-Györgyi's laboratory, discovered actin in 1942. Previously
Szent-Györgyi has shown that brief extraction of minced rabbit
muscle with 0.6 M KCl solution yields a myosin with low viscosity
(myosin A), whereas when the muscle mince was left in 0.6 M KCl for a
day a very viscous myosin solution was extracted (myosin B). Straub
thought that the difference between myosin B and A is caused by the
extraction of a new protein that makes the one-day extract viscous.
Accordingly, he extracted myosin A from the muscle, washed the
residue with distilled water to remove the KCl and remaining
cytoplasmic proteins, then dried the residue with acetone; the
residue contained mainly one protein and he named it actin. In
skeletal muscle, actin comprises about 15% of the total
protein.

The two forms of
actin: Water extraction of the acetone-dried muscle residue
yielded a protein solution with low viscosity, globular or G-actin,
that upon addition of salts (at physiological concentrations)
polymerized to a highly viscous actin gel, fibrous or F-actin. Straub
followed the polymerization by viscometry, shown in Fig. A1.

Fig. A1. Polymerization of
actin in the presence of various ions. Curve 1) 110 mM NaCl,
3 mM KCl, 3 mM CaCl2, and 10 mM MgSO4;
Curve 2) same as 1 but without Mg2+; Curve 3)
same as 1 but without Ca2+; Curve 4) same as 1
but without K+. Temperature 24o C
(From Feuer et al., 1948).

In 1949 Straub and Feuer reported that G-actin
contains bound ATP and during polymerization of actin the ATP is
hydrolyzed to bound ADP and Pi. Straub postulated that the
transformation of G-actin-ATP to F-actin-ADP plays a key role in
muscle contraction, however this could not be demonstrated in
skeletal muscle of live animals (Martonosi et al., 1960). Actin
polymerization with concomitant ATP hydrolysis may take place in
non-muscle cells and may provide the mechanochemistry for
motility.

Electron micrograph of fibrous actin filaments
reveals that the structure consists of twin strings of actin globules
wound around each other in a double helix. The subunit repeat is
about 55 Å and the helical repeat is about 370 Å.

Actin-myosin binding: F-actin combines
with myosin to form actomyosin. In 0.6 M KCl actomyosin forms a very
viscous solution resembling myosin B, extracted directly from muscle.
Upon addition of ATP, actomyosin dissociates into its components
actin and myosin, with accompanying reduction of the viscosity. At
physiological ionic strength actomyosin is insoluble, the same way as
in the muscle.

F-actin also combines with the proteolytic
fragments of myosin, HMM and S1. The complex formed
actoheavymeromyosin and actosubfragment 1 remains soluble at low
ionic strength. When HMM or S1 is added to muscle thin filament it
attaches to the actin component of the filament, forming a specific
"arrow head" structure (Fig. A3). This suggests a structural polarity
for the thin filament.

Based on this observation, H. E. Huxley
postulated that the structural polarity of thin and thick filaments
allows the sliding force to move the thin filaments toward the center
of the sarcomere (Fig. A4)

Three-dimensional
structure of actin: Kabsch and collaborators (1990) were the
first to crystallize G-actin and determined its structure (Fig.
A5).

Fig. A5. Scheme for the
structure of actin. (From Holmes and Kabsch, 1991).

Folding of the actin molecule is represented by
ribbon tracing of the a-carbon
atoms. N and C correspond to the amino- and carboxyl-terminals,
respectively. The letters followed by numbers represent amino acids
in the polypeptide chain. A hypothetical vertical line divides the
actin molecule into two domains "large", left side, and "small",
right side. ATP and Ca2+are located between
the two domains. These two domains can be subdivided further into two
subdomains each, the small domain being composed of subdomains 1 and
2, and the large domain of subdomains 3 and 4. (Subdomain 2 has
significantly less mass than the other three subdomains and this is
the reason of dividing actin into small and large domains). The four
subdomains are held together and stabilized mainly by salt bridges
and hydrogen bonds to the phosphate groups of the bound ATP and to
its associated Ca2+ localized in the center of the
molecule. Because of the less mass in subdomain 2, the actin molecule
is distinctly polar in the direction from subdomains 1 and 3, called
the "barbed end", toward subdomains 2 and 4, called the "pointed
end". This polarity defines the orientation of the actin molecule in
the myosin HMM decoration pattern of the thin filament, shown in Fig.
A3.

The intersubunit contacts in the F-actin
filament: In helical structures, such as the F-actin filament,
two types of intersubunit contacts are possible in principle: those
along and those between the long-pitch helical strands. In the atomic
model of the F-actin filament, 24 amino acid residues per subunit are
involved in contacts along the long-pitch helical strands. By
contrast, only 15 residues per subunit mediate the weaker contacts
between the two strands.

Localization of actin in the structure of
muscle: Under the microscope, myosin extracted myofibrils exhibit
the thin filaments, attached to the Z line. When 0.6 M KI solution,
that dissolves F-actin, is added to such a myofibrillar ghost the
structure disappears, indicating that the thin filaments are composed
of actin. In the structure of muscle, the I band contains thin
filaments whereas the A band.contains both thick and thin
filaments.

Structure of the thin filament: Fig. A6
shows the structure: actin molecules form two strings wound around
each other, in the grove is the tropomyosin strand and at regular
intervals troponin molecules are attached to tropomyosin.